338 BELL SYSTEM TECHNICAL JOURNAL 



It is repeated a sufficient number of times to establish a cathode 

 surface that is uniformly rough over its entire front surface and is 

 substantially the same for all cells. The exhaust system is then 

 evacuated and pure oxygen admitted to the optimum pressure for 

 quantitative oxidation. 



The quantitative oxidation is accomplished by charging a condenser 

 of known capacity to a definite voltage and then discharging it through 

 the cell in series with a suitable resistance. The discharge is in the 

 proper direction for oxidation of the cathode. A double contact 

 telegraph key is used for charge and discharge of the condenser. 

 The number of taps given to the key is then a quantitative measure 

 of the oxidation of the cathode. After the requisite oxidation of each 

 cathode, the system is again evacuated. 



As the oxidation proceeds, the surface goes through characteristic 

 changes in color. At 25 "taps" it is yellow, at 50 red, at 75 blue 

 and at 100 a greenish yellow. If the oxidation is continued, the color 

 goes through another cycle becoming a golden yellow, a deep rose 

 red and an olive green. From then on it is quite dark, but under 

 strong light shows several alternations of red and green before ending 

 in black. As will be seen later an oxide thickness corresponding to 

 about 100 taps is the most suitable for use. 



2. Preparation of Ccesium 



The early work demonstrated the necessity of a close control of the 

 amount of caesium made available for the cathode in each cell. After 

 an examination of a number of chemical systems for preparation of 

 caesium by a high temperature chemical reaction, a pellet composed 

 of a mixture of caesium chromate, chromic oxide and powdered alumi- 

 num was adopted. These materials are thoroughly mixed in quanti- 

 tative proportions and ground in an agate mortar. A suitable weight 

 of the mixture is compressed in a die. A slight but uniform loss of 

 material is entailed in the pellet making process. Any desired quantity 

 of pellets differing in weight by not more than ten per cent can be 

 made by this process. This pellet is placed in the molybdenum 

 housing described above and after the quantitative oxidation of the 

 surface and removal of excess oxygen the pellet is heated by high- 

 frequency induction to its kindling temperature. 



Sufficient aluminum is supplied to completely reduce the caesium 

 chromate and chromic oxide. Besides permitting a pellet of con- 

 venient size, the chromic oxide is instrumental in furnishing, by its 

 reduction, a large amount of heat. The high-frequency heating serves 

 only to initiate the reaction. The exothermic reaction involves a 



